Why merge all runtimes?

To ensure that everyone: app developers, gaia contributors and platform engineers are all on the same page and use the same environment and runtime.
Then merging all runtimes allows to share the advantages of all of them.
Gaia will run with the same feature set than b2g desktop (ie. many features require some glue code that doesn't exist in Firefox. for example system message API).
And we will have all devtools working against gaia.
While sharing the additional features coming from the simulator (rotation support for ex)

Where can I download it?

For Windows and Mac nightly builds are available here.
Look got firefox-* packages for your platform.

How different is mulet compared to the existing runtimes?

Compared to b2g desktop, mulet runs within a customized version of Firefox.
The goal is to seamlessly run b2g within a firefox tab.
So mulet contains all preferences, settings and glue code that used to exist in b2g desktop.

How do I build it?

Checkout mozilla-central and use "ac_add_options --enable-application=b2g/dev" in your mozconfig.

How do I run tests?

One exception are reftests: Mulet reftests run in a different environment than regular reftests. The mulet reftest window has chrome://b2g/content/shell.html as its root document and uses a gaia profile that contains the reftest extension. The Marionette test runner injects a script in the gaia environment that loads the reftest harness, which injects a (non-remote) xul:browser into shell.html.
The easiest way to run Mulet reftests locally is by using docker and executing the "Run locally" script from a taskcluster "Inspect task" page of a Mulet reftest job that has run on treeherder. See bug 1233206 for more information.

How to report bugs?

What is mulet for a gecko developer?

Mulet is a Firefox desktop build, with MOZ_B2G env variable set to 1.
This variable enables most of the b2g specifics that are built only on FxOS (and not on Fx Desktop).
It is also built with MOZ_MULET=1 in order to help distinguish Firefox Desktop from Mulet.
A dedicated folder exists for mulet in mozilla-central, m-c/b2g/dev, which contains mostly build/configuration files.